Electric-current-induced grain reorientation in Ni-stabilized η-(Cu, Ni)₆Sn₅ with stress-decoupled response

Ni-stabilized η-Cu₆Sn₅ is a key intermetallic in Sn-based solder joints, where high current densities impose strong electron-wind forces on grains and defects. Here, we use in-situ synchrotron Laue nanodiffraction and reciprocal-space mapping to quantify how Ni-stabilized η-(Cu, Ni)₆Sn₅ accommodates a steady current of 1.5 × 10³ A cm⁻² at ≈ 130 °C. The grain misalignment angle decays exponentially with time and is well described by a rotational drag model in which the electron-wind torque is balanced by grain-boundary viscosity, yielding a characteristic reorientation rate α ≈ 0.24 h⁻¹. During this evolution, all components of the Laue-derived deviatoric-stress tensor remain constant within the experimental resolution (≈ ±15 MPa), while reciprocal-space peak widths narrow after current removal but not under isothermal heating alone, indicating a non-diffusive, current-driven relaxation of defects and grain boundaries. In contrast, undoped η-Cu₆Sn₅ subjected to the same J–T conditions show only elastic strain storage with no measurable grain reorientation (α ≈ 0 within uncertainty). These results demonstrate that modest Ni additions activate a grain-boundary-sliding-mediated, stress-decoupled reorientation pathway, placing η-(Cu, Ni)₆Sn₅ in a “geometric-compliance” regime where grains realign at nearly constant deviatoric stress. This current-induced compliance has direct implications for the design of Ni-containing solder joints with improved resistance to electromigration-related damage.